By using first-person narration, Poe allows the reader to witness Montresor's thoughts. In doing so, the reader becomes aware of his possible insanity and the likelyhood of Montresor being an unreliable narrator. He withholds valuable information, lies to Fortunato throughout the story, and rationalizes his crime in a way no sane person would.

Sometimes people lie to themselves to justify their actions. You could argue that Montresor does feel guilt. The line about the bad air seems to indicate it to be possible. In that case, his arguments might be related to him trying to convince himself it was justified.

Well, you certainly have a good case for arguing this line. However, there are hints that show that Montresor might be affected more than he would have himself believe. Consider how he says at the end of the story that he shivers - and then rushes on to say that it was because of the cold. It is as if his body is reacting against the heinous crime he has committed but his mind is incapable of understanding his conscience - which is another proof that he is an unreliable and incredibly un-self aware narrator.